The dehydration of various materials by exposure to microwave radiation at reduced atmospheric pressures is well studied. See, for example, M. Zhang et al, “Trends in Microwave Related Drying of Fruits and Vegetables”, Trends in Food Science & Technology, 17 (2006), 524-534 (the entire contents of which is incorporated herein by reference). In general, a reduction in atmospheric pressure reduces both the boiling point of water and the oxygen content of the atmosphere. Vacuum microwave dehydration, VMD, processes may accordingly permit dehydration to occur in the absence or reduction of oxygen, and without exposing the material that is being dehydrated to significantly elevated temperatures, thereby yielding dried products that may have better physical, organoleptic and/or chemical qualities as compared with dried products obtained using other known dehydration processes such as hot air convection or freeze drying. VMD processes may also be relatively quick and energy-efficient as compared with many other dehydration processes. Temperature and/or oxygen sensitive materials of the sort that are known to be amenable to drying by VMD include, but are not limited to, food products such as fruits, vegetables, berries, herbs, meats, fish, seafood, dairy products, prepared foods, seeds, grains, roots and tubers, as well as a wide variety of agricultural feed products, pharmaceutical and nutraceutical products, dietary supplements, synthetic organic compounds, and the like.
As is well known, VMD may be carried out as a batch or continuous process, and a typical VMD apparatus will comprise at least a vacuum chamber (in which an input material is dehydrated into an end product), a source of microwave radiation, and associated sensing equipment (e.g. infra-red detectors) and control equipment (e.g. a programmable logic controller, “PLC”) to monitor the status of the product during the dehydration procedure and to make desired or necessary adjustments. For example, such monitoring may include monitoring the surface temperature of the material (such as by using infrared detection) or surface texture (e.g. wrinkling). In continuous VMD processes, the apparatus will also typically comprise input and output means such as air locks that permit the input material and end product to enter and exit the vacuum chamber, respectively, without disrupting the vacuum level, and a conveying means (e.g. a conventional conveyor belt) to convey the material through the vacuum chamber between the input and output ends.
It has generally been established in relation to known VMD processes that a higher microwave field strength will have a greater effect (as measured over the complete drying cycle) on increasing the rate of dehydration than does a deeper vacuum. A primary focus of current state of the art VMD apparatus and process engineering has accordingly been to maximize the intensity of microwave radiation that can be applied to the material being dried.
In keeping with the general objectives of maximizing microwave field intensity while controlling the temperature gain of the material being dried, the microwave emitters (e.g. magnetrons) of current VMD apparatuses are typically located outside of the vacuum, or irradiation chamber where they may be operated under atmospheric conditions (and protected from the conditions within the chamber). The microwave radiation generated by the emitters enters the vacuum chamber through one or more microwave-transparent windows typically after being conveyed through one or more waveguides. Various microwave waveguides are known in the art. Non-gas dielectric waveguides include microstripline, coaxial, and stripline types. However, such dielectric waveguides convert some of their energy into heat (i.e. are “lossy”), and typically cause microwave fields to be established on the outside surfaces of the waveguide. For most microwave applications, this results in microwave radiation interacting with anything that happens to be near the dielectric waveguide. For these reasons, the waveguides used to convey microwaves from the emitter to the irradiation chamber are generally also maintained outside of the chamber. Such placement serves to reduce the occurrence of high voltage standing waves caused by reflection of microwaves, which may lead to arcing within the waveguide. Thus, in typical known microwave dehydration apparatuses and methods, the material to be dehydrated is generally subjected to microwave radiation in the far-field region.
In general, as is known in the art, the power density in the electromagnetic far-field region is reduced as the square of the distance from the source. However, within the near-field region (i.e. a distance that is within about one wavelength of the electromagnetic radiation, but possibly extending so far as to include a transition zone ending within about two wavelengths), very high electromagnetic fields that do not decrease as the square of the distance may occur. This enables relatively high field strengths to be developed within the near-field region.
There exists a need for an improved apparatus and/or method for dehydrating materials, such as food products and the like, using microwave radiation that overcomes at least one of the deficiencies known in the art.